1. Field of the Invention
The present invention relates to a storage control program, a storage control apparatus, and a storage control method for writing a log onto a storage apparatus.
2. Description of the Related Art
A hard disk causes a random access to be slower than a sequential access. Storage systems using hard disks are subject to the similar problem. Several methods are proposed to solve this problem.
A first method is to simply array a large number of disks for performing I/O operations in parallel and improving the performance. This method can be used to construct a large-scale RAID (Redundant Arrays of Inexpensive Disks) system and improve both IOPS (Input Output Per Second) and reliability of random access.
A second method is to use a cache. This method can be categorized into write through cache and write back cache. The write through cache uses the cache during reading. The write back cache uses the cache during reading and writing. These methods can be used with an increased cache capacity to increase a cache hit ratio and drastically improve the IOPS for random access.
The write back cache needs to save data in memory before writing the data to a disk, thus degrading the reliability. Multiplexing the cache may ensure the reliability but doubles the cache memory cost.
A random write will now be described.
The random write includes a method called a logging operation. The logging operation sequentially writes information about a logical position (address) and data for the random write. Data reading uses information in a position information table that indicates correspondence between logical and physical positions to read data for reproduction. In this manner, the random write can achieve a throughput comparable to that of the sequential access. The logging operation excels in the random write. The write through cache excels in the random read. A combination of the logging operation and the write through cache can provide preferable results in terms of the performance, reliability, and costs for the random access.
Conventional technologies related to the invention disclose a storage system that distributes a cache apparatus, a disk apparatus, and a control apparatus over a network to increase a cache memory area. (For example, see Patent Document 1: WO2003/065195, Patent Document 2: WO2003/075147, and Patent Document 3: WO2004/027625).
The combination of the logging operation and the write through cache is very effective but causes a problem specific to the logging method. Especially, a storage system based on the logging operation needs to manage a position information table that indicates the correspondence between a logical position, i.e., a write position used by a host, and a physical position, i.e., a write position for a low-order device (disk apparatus). Searching the position information table is time-consuming, thus degrading the reading performance in case of a cache miss. The position information table necessitates one-to-one correspondence between sectors and requires an extra space approximately 1/64 to 1/512 of a volume size.